I stand firm on my comments due to experience.

Manufacturer's suggest an installation for many reasons. Mainly for best results and secondly, for unnecessary returns.

Excluding pumped engines, an engine seems to run best when it is forced to draw fuel ie: tank outlet slightly lower than carb. inlet.

More on the plane I had problems with: First, with the tank mounted way above the carb, not only did it not idle properly, overall throttle operation was erratic. At this point with a new plane never flown, it remained on the ground. All basics were then performed: Plumbing check including inside tank, cylinder head and rear crankcase cover bolts, glow plug, fuel, carb. mount O ring, muffler...etc.

After the next attempt: same results=N.G.

Repeated all the above= still N.G

Back home to the "drawing board"!

Re-located tank= JOY!

An engine creates more vacuum at higher throttle setting, therefore, it should pull fuel from a greater distance or even an improperly located tank.

There is no siphoning of fuel at all when the engine (In this case an OS91) is not in use. However when assembling the plane at the field, the glow plug is always removed and the engine spun to check if siphoning did occur.

Maybe relocating the tank was a solution specific to only this particluar OS91...but I honestly don't think so.